X-ray binaries are some of the strangest ducks in the cosmic zoo, attracting attention across thousands of light-years. Now, astronomers have captured new high-resolution radio images of the first star ever discovered, named Circinus X-1. Their images show a strange type of jet emanating from the binary's neutron star. The jet rotates like an off-axis sprinkler as it spits material out into the surrounding space and sends shock waves through the interstellar medium.
The MeerKAT radio telescope in South Africa discovered the S-shaped jets emanating from the neutron star. Its images are the first high-resolution images of such jets, according to lead researcher Fraser Cowie. “This image is the first time we have seen strong evidence of a precessing jet from a confirmed neutron star,” he said, referring to the neutron star's off-axis rotation. “This evidence comes from both the symmetric S-shape of the radio-emitting plasma in the jets and the fast, broad shock wave that can only be generated by a jet changing direction.”
Such unfavorable rotation gives the jets their peculiar S-shaped configuration. Since scientists aren't entirely sure what phenomena caused them to launch in the first place, studying the strange behavior sheds light on the extreme physics behind its existence.
Studying the jets of neutron stars in detail
The MeerKAT measurements showed not only the jet, but also termination shocks moving away from the neutron star. These occur in regions where the jets encounter material in the surrounding space. This is the first time astronomers have found such shock waves around an X-ray binary such as Circinus X-1. These waves move quickly – at about 10 percent of the speed of light – and their structure points to the jet as the source. “The fact that these shock waves cover a wide angle is consistent with our model,” said Cowie. “So we have two strong pieces of evidence that the neutron star jet is developing.”
A MeerKAT radio image of the S-shaped jet precessing in the X-ray binary system Circinus X-1. The jet is formed as a result of material accretion around the neutron star. Image courtesy: Fraser Cowie, Attribution CC BY 4.0.
The speed of these shock waves turns them into particle accelerators that produce high-energy cosmic rays. The fact that this radiation exists tells astronomers that the activity around the X-ray binary is extremely energetic. This high-energy activity has captured the attention of astronomers for half a century. Still, it remains a mysterious system, and so, as Cowie points out, it is important to observe the jets and see how their behavior changes over time. “Some aspects of its behavior are not well explained, so it is very rewarding to help shed new light on this system and build on 50 years of work by others,” he said. “The next steps will be to continue monitoring the jets and see if they change over time as expected. This will allow us to measure their properties more accurately and continue to learn more about this enigmatic object.”
About Circinus X-1
The Circinus X-1 system contains a neutron star and a companion. The pair lies about 30,000 light-years away in the direction of the constellation Circinus in the Southern Hemisphere. It was first discovered in June 1969 by a suborbital Aerobee rocket carrying X-ray-sensitive instruments, and has been studied by astronomers for years using optical, X-ray and radio telescopes.
Composite image of Circinus X-1, located about 24,000 light-years from Earth in the constellation Circinus. Image credits: X-ray: NASA/CXC/Univ. of Wisconsin-Madison/S. Heinz et al; Optical: DSS; Radio: CSIRO/ATNF/ATCA
The system is a fairly young member of the class of X-ray binaries. Typically, a binary pair consists of a black hole and a sun-like star, or a neutron star and a sun-like star. The enormous gravity of the more massive member of the pair continuously pulls material away from the companion. Eventually, a hot disk of gas forms, which spirals down to the surface of the neutron star. This accretion process releases enormous amounts of energy, and some of it drives the jets. They transport material away from the system at almost the speed of light.
Image of the Circinus X-1 and its jets.
Circinus X-1 is about 4,600 years old, based on studies of the material surrounding the binary system using the Chandra X-ray Observatory. It is one of the brightest objects in the X-ray sky and has been studied since its discovery. The neutron star component is extremely dense and is the leftover neutron-rich core of a supermassive star that exploded as a supernova about 4,600 years ago.
Astronomers know of hundreds of X-ray binaries in the Milky Way alone. Studies of Circinus X-1 give them insight into events and processes that take place in the early phase of the binary. Interestingly, another X-ray binary shows an S-shaped jet structure. It is called SS 433. However, it may not contain a neutron star. Instead, there could be a black hole that supplies this system with energy. This makes the existence of Circinus X-1 doubly interesting, as it contains a neutron star that does essentially the same thing.
For more informations
Jet like a garden sprinkler system shoots out of a neutron star
X-ray binary star Circinus X-1
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